1. Field of the Invention
The present invention relates to communication networks. More specifically, the present invention relates to methods and apparatus for Time Division Multiple Access (TDMA) communication network protocols.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Communications systems employed for short range communications between a reader transponder and a vehicle transponder are known in the art. Existing communications systems employed to communicate with and identify specific vehicles at certain locations require an antenna to be positioned in each traffic lane of the roadway. Each of the antennas are either connected to a dedicated roadside reader transponder or are multiplexed to a single reader transponder. These systems are designed to communicate with only a single vehicle per traffic lane and then only if the lane is equipped with an antenna. Further, the existing systems require each vehicle to slow down or stop, or to remain in a particular traffic lane during the identification process of the vehicle.
An example of a single traffic lane communications system known in the art includes a roadside reader transponder and a vehicle transponder. The system employs lane side or overhead antennas in each traffic lane and exhibits a read-only capability. The reader transponder senses the presence of the vehicle either by sending a periodically transmitted RF signal or by employing a vehicle sensor. Next, the reader transponder transmits an unmodulated carrier wave to the vehicle transponder. The carrier wave is a high frequency RF signal, e.g., a tone. If the unmodulated carrier wave is of the proper frequency and exceeds a threshold power level, the vehicle transponder responds by modulating the carrier wave. Thereafter, the method of modulated backscatter know in the art is employed in the vehicle transponder to transmit 128 bits of information back to the reader transponder.
The RF trigger signal must be controlled in order for this system to function properly. Additionally, the vehicles being monitored must slow down or stop and only a single vehicle per traffic lane can be monitored at any time. Further, barriers must be erected between traffic lanes to control the field of the RF trigger signal. If two vehicles each equipped with a transponder are in range of the RF trigger signal, each transponder will respond. This situation results in destructive interference of the two response signals. The interference is caused by the two response signals, one from each vehicle transponder, returning to the reader transponder simultaneously. The two signals become superimposed resulting in ambiguity in the received information which can be, for example, the identification of each of the vehicles.
A second example of a known communications system also includes a reader transponder and a vehicle transponder. This second communications system employs in-pavement antennas in each traffic lane and utilizes a transmitted RF trigger signal or vehicle sensor to detect the presence of the vehicle. If the RF trigger signal transmitted by the reader transponder satisfies the frequency and threshold power level requirements, the vehicle transponder responds with an identification message. Although the data capacity is comparable to that of the first communications system example described above, the data rate is much higher. This second communications system example is also prone to destructive interference when multiple signals are returned from more than one vehicle transponder. When the signals are superimposed, the response becomes garbled and senseless.
Clearly, both of these communications systems have shortcomings associated therewith. The infrastructure costs are high since many additional components (such as antennas, transceivers, barriers, etc.) are required to support the system. The restrictions on traffic flow (such as the requirements to slow down or stop, or to remain in the same traffic lane) are burdensome. Further, the reliability of the system is reduced during high traffic density periods or when vehicles straddle a traffic lane. Additionally, the read-only capability limits the prior art communications systems to toll collection applications. Unfortunately, read-write and broadcast capabilities are not available in these conventional systems.
Two TDMA communication network protocols known in the art are the plain aloha/pure slotted aloha scheme and the fixed slotted assignment scheme. In the plain aloha scheme, users of the network transmit messages at randomly selected time intervals. When few messages are being transmitted, all of the messages are successfully received. However, as the number of users increases, the messages tend to overlap and interfere with one another. In the pure slotted aloha TDMA scheme, users pick time slots at random in each frame. The selection of the time slots and the transmission of the data is controlled by the software within a modem. In the slotted aloha scheme, the message transmission can occur only within a specified time slot and is not permitted to straddle a time slot. If two users select the same time slot, the signals will collide and both messages are lost. Thereafter, users continue to select time slots until successful communication is achieved. When the network is employed to monitor vehicles on a freeway, the data throughput becomes excessive and the slotted aloha scheme becomes undesirable.
An alternative TDMA communication network protocol is the fixed slot assignment scheme. A modem is designed to implement a specific protocol and in this protocol, specific time slots are fixed for specific users. Therefore, the modem transmits data in fixed slot assignments for the specific users. However, the fixed slot assignment protocol becomes very inefficient when slots must be reserved for a large number of potentially infrequent users. Once a vehicle has passed a roadside reader transponder, a slot reservation for that vehicle need no longer be retained. However, difficultly arises in predicting when a particular vehicle will return to communicate with the reader transponder.
Thus, there is a need in the art for improvements in TDMA communication network protocols and the transponders associated therewith.